The use of lithography for creating very fine patterns is limited by the dependence of the limit of resolution and depth of focus on the illumination wave length. In addition to optical lithography, electron beam and x-ray lithography have therefore been suggested as alternatives because of their shorter wave lengths. However, up-to-date production procedures require very high levels of capital in attempting to extend the use of optical lithography to smaller structures.
In executing a typical photo-lithographic process for a semiconductor device, a mask is used whose pattern is transferred to a wafer. The use of masks which incorporate a phase shifting element (in the following called a phase shift mask) is known in photolithography for reducing the critical dimension while simultaneously improving the resolution. One disadvantage of the prior art phase shift masks is that the lines created do not have sufficiently defined areas at the ends of the lines. In addition, the manufacture of prior art phase shift masks is laborious and costly.